Abstract: An endoscope provided with a stop comprising an annular electrochromic element arranged in an objective lens unit so that the transmittance of the stop is varied in accordance with the amount of light from an object to be observed which has been measured, in order to prevent brightness in a field of view from being rapidly changed when the distance from the distal end portion of the endoscope to the object to be observed is varied.

Abstract: A single-lens reflex optical system for endoscopes comprising an observing optical system, a light-splitting means and a photographing optical system, the observing optical system comprising a diverging front lens group arranged on a first optical axis extending in parallel with the longitudinal direction of an endoscope and having negative refractive power, a positive lens group arranged on the first optical axis, and a first converging rear lens group arranged on the first optical axis and having positive refractive power, the light-splitting means arranged between the positive lens group and first converging rear lens group, the photographing optical system comprising the diverging front lens group, the positive lens group, and a second converging rear lens group arranged on a second optical axis deflected by the light-splitting means and having positive refractive power, the single-lens reflex optical system enabling to make diameters of lenses small and, consequently, enabling to make the diameter of dis

Abstract: An objective for endoscopes to be used in such endoscope arranged to transmit an image of an object by a plural number or relay lens groups and basically comprising a front diverging lens group, which comprises a negative lens, and a rear converging lens group, which comprises a positive lens and positive cemented doublet. The objective for endoscopes is arranged so that aberrations of the endoscope as a whole including the relay lens groups are corrected favorably.

Abstract: A forward-oblique viewing optical system for endoscopes comprising an observing-direction changing prism which has a surface for refracting rays from an object to be observed and is arranged in front of an objective, said optical system as a whole thereby being made compact.

Abstract: A single-lens reflex optical system for an endoscope comprising a first prism having a reflecting mirror with a small hole located at a position near the aperture stop of the photographing optical system, a second prism cemented to the first prism, and an observing optical system arranged near the exit surface of the second prism. In the above single-lens optical system, the aperture stop of the photographing optical system and aperture stop of the observing optical system are respectively arranged at angles close to a right angle in respect to the corresponding optical axis.

Abstract: A retrofocus-type objective for endoscopes comprising a front diverging lens group and rear converging lens group, said rear lens group being divided into a first lens group and second lens group, said retrofocus-type objective being arranged to be focused by varying the airspace between said first and second lens groups. By the above arrangement, the front lens is kept fixed and serves also as a cover glass so that the retrofocus-type objective as a whole becomes small in size.

Abstract: A retrofocus-type objective for endoscopes comprising a front diverging lens group and rear converging lens group and arranged to be focused by keeping the front diverging lens group fixed and moving the rear converging lens group toward the front diverging lens group.